In another essay, I introduced the concept of the Law of Balance, the idea that, in the macroscopic world of predators and prey, there exists a balance between all living organisms, so that no single species is able to dominate over others. In this essay, I wish to consider whether the Law of Balance also applies to the microscopic world of bacteria, algae, viruses, and other organisms that are visible to us only when we view the world through a microscope.
Like many scientists who study the ancient history of Life on Earth, I believe that, before there was a world of macroscopic creatures – the creatures that we are familiar with in our lives, such as mammals, birds, fish, insects, reptiles, amphibians, crustaceans, and so forth – the Earth was first populated by microscopic organisms like bacteria and algae. What were the relations that existed between these microscopic creatures when they were the only living organisms on the planet? In my opinion, there is a close analogy between the effects of the Law of Balance at the macroscopic level and the microscopic level. In a sense, disease is the interface at which these two worlds meet, since disease, which results in the death of macroscopic organisms, is caused by the unchecked multiplication of microscopic organisms that feed or live on them.
A single microbe, even if it is a microbe that causes bubonic plague, cholera, tuberculosis, smallpox, polio, diphtheria, dengue fever, sleeping sickness, influenza, malaria, or AIDS, cannot cause any harm to a living organism. Only if it is able to replicate and multiply into very large numbers can it cause the organism’s death. Clearly, this is what happens in the case of a person whose immune system is overwhelmed by the multiplying microbial hordes, which debilitate the person’s body, causing sickness, infirmity, or death.
I suppose there are many people who have seen, such as in a video or television program, the rapid unchecked multiplication of a deadly virus or bacterium that is cultivated in a laboratory. Due to images like these, there are many people who have a general fear of all microbes. These people, called germophobes, believe that they must take constant precautions to protect themselves from potentially deadly microbes, which may lurk unseen, anywhere and everywhere in their environment, waiting to infect their vulnerable bodies, thus making them sick or, in extreme cases, even causing their death.
However, just as animals that are bred in captivity are not eaten by predators – apart from humans, of course – since they are artificially protected from all potential predators, microbes that are bred in perfectly sterile laboratory conditions with an abundant food supply, multiply without check because this is a completely unnatural environment. To see what happens to them in the real world, where there are many other kinds of microbes with which they must compete for survival, including some that may eat them or prevent them from multiplying in other ways, such as by producing substances like alcohol or lactic acid that are toxic to them, they must be studied in their natural environment. In other words, in all normal – meaning non-laboratory – conditions, microbes do not exist in a microbial vacuum where they can multiply unchecked. Just like all macroscopic organisms, which exist in an interconnected and interdependent ecosystem, microbes also exist in a microbial ecosystem in which their numbers are usually kept in check. Similarly, at the macroscopic level, apart from humans, no single species of organisms is able to dominate its environment, to the detriment of all other organisms, in accordance with the Law of Balance.
There are many germophobes who mistakenly believe that they should try to replicate the sterile conditions that exist in laboratories on their bodies and in their daily lives. But such a goal is impossible, for, outside of laboratories, there are microbes everywhere – in the air, water, and soil, and also on and inside our bodies. Moreover, an organism that is completely devoid of microbes would be almost certain to die if it is exposed to the natural, meaning non-sterile and microbially-rich, environment that exists outside of laboratories.
It’s possible, Vance says, to breed sterile—as in germ-free—mice, ones with absolutely no gut biota. These mice live in sterile cages, eat special food, and breathe filtered air. If they are taken out of their cages, their immune systems collapse when exposed to the meekest pathogens, and the mice die.
Those who deal regularly with certain kinds of microbes, such as those who bake sourdough bread, brew beer or ferment fruits, grains, or other sugars and carbohydrates to produce other kinds of alcoholic beverages, make cheeses using traditional methods, or ferment foods naturally, understand that they are attempting to cultivate microbial colonies in order to transform these foods by natural processes. These colonies are just as complex, albeit on a smaller scale, as the macroscopic ecosystems that many people are more familiar with, whether on land or in the air or water.
Perhaps the USDA team’s most important contribution was to demonstrate that a sourdough culture functions as a kind of ecosystem, with the various species performing distinct roles that lend stability to the culture over time. Once established, the system exhibits more cooperation than competition, so that no one organism ever dominates. Subsequent research in other parts of the world has greatly expanded the list of species found in sourdough cultures—at least twenty types of yeast and fifty different bacteria—but most of them seem to fall into similar niches, organize themselves into similar relationships, and perform similar functions. Same play, different actors. Presumably these yeasts and bacteria coevolved with one another, which might explain why many of them have been found nowhere except in sourdough cultures, their “natural habitat.”
There are bakers who have used the same sourdough starter, a mixture of flour and water that supports a complex living microbial colony, for many years, without it going bad due to the proliferation of unwanted microbial species; and this is not because the starter is protected from unwanted microbes, such as by being sterilized or irradiated, since these processes would kill all microbes, both desired and undesired. What this shows is that the presence of these many different kinds of microbes, which probably consists of a combination of yeasts, fungi, and bacteria, prevents the proliferation of other microbes that would also like to consume the finely pulverized wheat.
Human beings have been making cheeses for millennia. In the past, they did so with naturally-occurring microbes in what would now be considered to be highly unsanitary conditions. In contrast, today, many industrially-produced cheeses are made in stringently sanitary conditions where the microbes that are allowed to ferment the curds that are separated from the whey when milk is coagulated are strictly controlled, which often results in a finished product that is bland and lacks the rich, complex taste of a naturally-fermented cheese. Although spoilage does sometimes occur, which means that the cheese has been colonized by microbes other than the ones that one wants to be present in the cheese, in the majority of cases, this process occurs without spoilage. In other words, during the process of fermentation, the colonization of the cheese by the desired microbes prevents harmful or undesired microbes from gaining a foothold and multiplying. And the same process probably takes place in the case of an organism’s health: the existence of a flourishing microbial colony both on and inside the organism helps to protect the organism from potentially dangerous microbes, by preventing them from gaining a foothold and multiplying.
There are important similarities between invasive macrospecies and invasive microspecies. From numerous unfortunate experiences, it is now known that the introduction of a new species, whether plant or animal, in an area where it had previously not existed can lead to its unchecked multiplication if it lacks any predators, or organisms that consume it. This is the reason for the strict border controls when people travel from one part of the world to another, or when they attempt to send seeds, plants, or animals from one country to another. For example, both rabbits and cane toads have multiplied largely unchecked in Australia, where they were introduced deliberately, the first as a source of meat and sport hunting, and the second as a form of pest control. Similarly, certain species of carp have multiplied and become a serious problem in the Mississippi River, from which they have begun to migrate to other contiguous bodies of water.
The same effect that is visible in the case of what is called an invasive species – a macroscopic species that is able to multiply unchecked, limited only by its food supply, due to an absence of predators in its new environment – is also visible in the case of a disease-causing microbe that is introduced to a population, whether human, animal, or plant, where it had formerly not existed. It is now estimated that, following Columbus’ “discovery” of the Americas, between seventy and ninety percent of the native people living on these continents and islands were killed, most of them unintentionally but in some cases intentionally, by the introduction of diseases to which Europeans were more or less immune, but to which the natives were not immune, never having encountered them before.
The general belief why a deadly disease does not eradicate an entire population is that some individuals possess immunity to the disease, which may be due to genetic variability, which immunity may then be passed on to their offspring. This is an example of what has been called “the survival of the fittest,” in this case, fitness meaning the organism’s ability to defend itself against a malicious microbe that can cause its death. However, this belief is not proven, and I suspect there is another important mechanism that protects people, and organisms in general, from deadly microbes.
I wish to emphasize that I am not questioning the standard medical belief that diseases are caused by specific bacteria or viruses that multiply unchecked inside the human body, thus causing sickness or death. According to the germ theory of disease, the human body’s immune system destroys potentially harmful microbes, or germs, and this is what protects people and keeps them in good health. When it is unable to do so, disease results, with all its unpleasant effects, including death. Although I agree with this theory, I believe that, in a very fundamental way, it provides an incomplete picture of how bodily health is maintained.
You would think that his [Elie Metchnikoff’s] brain, which seemed to be able to hold all knowledge, would have dreamed of subtle tests to find out just how it is that one child can be exposed to consumption [tuberculosis] and never get it, while some carefully and hygienically raised young girl dies from consumption at twenty. There is the riddle of immunity (and it is still completely a riddle!).
The belief that human health is due solely to the body’s ability to produce white blood cells and antibodies to destroy all the dangerous microbes that can harm or kill it is only part of the picture. For just as all healthy ecosystems require a wide variety of different organisms in order to survive and remain in optimal health, human bodies also require a wide variety of different microbes in order to survive and remain in balance.
In Cooked, Michael Pollan describes the traditional cheese-making technique of Sister Noëlla Marcellino, who learned how to make an ancient cheese called Saint-Nectaire from a Frenchwoman named Lydie Zawislak, whose family has been making the cheese for generations. Instead of a sanitized stainless steel vat, which is the standard equipment used by most commercial cheese-manufacturing companies, she makes the cheese in a wooden barrel and stirs the curds with a wooden paddle.
And indeed the inside of Sister Noëlla’s cheese-making barrel wears a permanent cloak of white—a biofilm of milk solids and bacteria. You could not completely sterilize it if you tried, and part of the recipe for Saint-Nectaire involves not trying: Lydie told Noëlla that between batches the barrel should only be lightly rinsed with water.
So it happened that in 1985, after raw-milk cheese was implicated in the deaths of twenty-nine people in California, the state health inspector demanded that Sister Noëlla get rid of her wooden barrel and replace it with stainless steel.
Sister Noëlla regarded her wooden barrel and paddle not merely as quaint antiques, but as essential elements of the traditional cheese-making process. The fact that the wood harbored bacteria was actually a good thing. She preferred to think of them not as contaminants but “more like a sourdough culture.” So Sister Noëlla designed an experiment for the benefit of the health inspector. From the same raw milk, she made two batches of cheese, one in the wooden barrel, and the other in a stainless-steel vat. She deliberately inoculated both batches with E. coli.
What happened next was, at least to a Pasteurian [someone who believes that most microbes are bad, and that the germ theory of disease tells the whole story of human sickness and health], utterly baffling: The cheese that had been started in the sterile vat had high levels of E. coli, and the cheese made in the wooden barrel had next to none. Just as Sister Noëlla had expected, the “good bacteria” living in the barrel—most of them lactobacilli—had outcompeted the E. coli, creating an [acidic] environment in which it couldn’t survive. As had happened in my [naturally-fermented] sauerkraut, the good bugs, and the acids they produced, had drive out the bad. The community of microbes in the raw-milk cheese was, in effect, policing itself.
Clearly, it was easier for the E. coli to multiply in the stainless steel vat because it had fewer microbial adversaries with which it had to compete for survival.
There is a similarity between microbes and languages. In an uninhabited or sparsely inhabited place, which is similar to the sterile conditions that exist in a laboratory dish used to culture microbes, the language spoken by explorers or by the first settlers can become dominant if they are able to multiply their numbers so they remain the dominant linguistic group. But in a place where there are many speakers of a certain language, the introduction of one or a few foreign speakers will, in the long run, not have any effect on the linguistic balance. Chances are, these individuals, and in particular their children, will become assimilated to the dominant language, so that they will cease speaking their native language. For example, there were millions of German speakers who immigrated to the United States in the past. However, the great majority of their descendants no longer speak German, having been assimilated into the dominant English-speaking majority, which preceded them and proceeded to multiply in these lands. Of course, the situations are not identical, since microbes do not transform other microbes into their kind, but rather prevent them from multiplying or cause them to die, as lactobacilli do by producing lactic acid, or some yeasts do by producing alcohol.
There are approximately 700 different kinds of bacteria that have been found in people’s mouths. Of course, a single person has a much smaller number of these oral bacteria, with estimates ranging from less than a hundred to between one hundred and two hundred. Bacteria require three things in order to thrive: they require moisture, food, and an ideal ambient temperature, all of which conditions are amply provided by a person’s mouth, since food regularly enters it and remains there while it is being chewed. Moreover, small quantities of food, which are more than sufficient to support bacterial growth, may get stuck in various parts of one’s mouth and remain there long after the rest of the food has been swallowed. Different bacteria thrive at different temperatures, but there are many bacteria that thrive at normal human body temperatures, including those that cause human diseases.
Let us also remember that, for the vast part of human history, our ancestors did not wash or sterilize their food, such as by cooking it over fire. In other words, the things they put in their mouths, which later passed into their stomachs and intestines, were probably loaded with microbes, including potentially harmful microbes. This is also true of all other animal species, since none of them bother to clean or cook their food before eating it – at least those that live in the wild, since many human pets are also fed cooked or sanitized, meaning microbially deficient, food. Of course, some of these distant human ancestors probably did become sick or die as a result of their unsanitary eating habits. But the question is, if the germ theory provides a complete picture of human disease and health, why did they not all become sick and die, so that the human species, as well as all other animal species, became extinct due to their disgustingly unsanitary feeding habits?
I believe the answer has to do with the large number of different kinds of bacteria that exist in a person’s mouth, as well as in one’s gastrointestinal tract. Clearly these bacteria do not harm their host, since otherwise a person would soon succumb to one or more of them. Just as in the cheese-making illustration, besides other functions, such as breaking down food to make it more easily digested, what these bacteria do is to make it difficult for any foreign microbes, including those that cause disease, to gain a foothold and multiply so that their teeming numbers can harm, debilitate, or kill the host organism.
There is also the significant fact that, since we must chew our food before swallowing it, unlike other creatures that swallow their food without chewing it, such as seals, snakes, penguins, owls, whales, and many predatory fish, it sometimes happens that we accidentally bite our tongue or the inside of our mouth while we are chewing. And yet, these minor injuries almost never lead to infection, even though we do not take the precaution of disinfecting the damaged skin in order to protect it from microbial infection.
Indeed, the species of bacteria living on your left hand are different from those living on your right.
If the bacteria that exist on different parts of the same person’s body are dissimilar, then this means that it is also possible that even individuals who live in close and intimate proximity, such as the members of a family or tribe, may also have different kinds of bacteria living on and inside them. Hence, this provides a possible answer to the riddle of why some people are not affected by a deadly new disease, while others succumb to it rapidly: because the bodies of those who are immune to the disease possess a microbial environment that makes it difficult or impossible for the new microbe to multiply into the large numbers that it requires in order to overwhelm the host organism. For we must not forget that a single microbe, or even a relatively small number of them, can do no harm to a large organism like a human being. It is only when they are able to multiply unchecked that they can cause sickness and death.
Another practice that is common today but historically was completely unknown among our ancestors is drinking very hot liquids. Even after some humans mastered the art of using fire to warm themselves, as protection against predators, to light their way at night, and perhaps to cook their food, it was still a long time before some of them acquired the taste for drinking hot liquids, since this requires the additional complex art of making fireproof and waterproof ceramic or metal vessels to hold the liquid. If we survey the animal kingdom, we will find that, in the wild, there is no other animal that enjoys drinking water, or some other beverage, that is almost boiling hot. As most people know, heating something, whether liquid or solid, to a high temperature is one method of sterilizing it and killing any microbes it contains.
That many people are more susceptible to colds and sore throats during the winter may actually be due to another cause, namely the fact that, in many countries around the world, people tend to drink more hot liquids during the winter than they do during the warmer months. This practice may inadvertently deplete the microbial colonies in people’s mouths, throats, esophagus, and stomachs, making them more vulnerable to viral and bacterial infections. According to the germ theory of disease and infection, the loss of these microbes makes no difference to a person’s immunity and health, since the fewer the microbes on or inside one’s body, the better. But according to what I will call the Healthy Microbial Ecosystem, or HME, hypothesis, the loss of these microbes do matter.
When one consumes something that is very hot, such as tea, coffee, or soup, the hot liquid first enters one’s mouth, passes down one’s throat, and then enters one’s stomach. Along the way, it gradually cools down until it reaches the same temperature as the inside of one’s body, whose temperature has been raised slightly by the excess heat energy contained in the liquid or food. Viewed from the microbial perspective, the ingestion of this hot liquid is probably a disaster, comparable at the macroscopic level to a fire that destroys parts of a forest. The bodily regions that are most at risk of having their microbial colonies decimated or reduced by this common custom are the lips, mouth, and throat. Since, according to the HME hypothesis, the body’s microbial colonies protect people from potentially harmful microbes by preventing them from multiplying rapidly, this leads to the testable hypothesis that those who regularly ingest very hot liquids will be more susceptible to contracting common ailments such as colds, flus, and sore throats, and perhaps other infectious diseases.
Some other common habits that may deplete or alter a person’s microbial ecosystem are taking antibiotics, smoking, drinking chlorinated water, and ingesting substances that contain a high percentage of alcohol, since all of these substances – antibiotics, smoke, chlorine, alcohol, and mouthwash – can kill microbes. By regularly inhaling smoke through their throat and into their lungs, smokers may be killing some of the microbes that exist in these regions, which may explain why they are more susceptible to respiratory infections. Even though humans have been consuming alcoholic beverages for millennia, the practice of distilling, which can greatly increase a beverage’s alcoholic content, and hence its toxicity, is a more recent discovery.
The overly simplistic picture of human health that results from the isolationist scientific approach to understanding things leads to a very seriously incomplete picture of bodily health. For according to the germ theory of disease and health, all microbes are either bad or, at best, innocuous, and therefore, the more microbes one eradicates, the healthier one will be. But according to the holistic approach, which does not deny the truth of the germ theory, but supplements it with additional important information, the existence of a flourishing microbial ecosystem is also necessary for good health, for these microbes help to prevent harmful microbes from establishing themselves in an organism and multiplying unchecked, thus making it sick and possibly causing its death. This is the opinion of Sandor Katz, who, based on his personal experiences, has become a fervent advocate of eating naturally-fermented foods:
“To declare war on ninety-nine percent of bacteria when less than one percent of them threaten our health makes no sense. Many of the bacteria we’re killing are our protectors.” In fact, the twentieth-century war on bacteria—with its profligate use of antibiotics, and routine sterilization of food—has undermined our health by wrecking the ecology of our gut. “For the first time in human history, it has become important to consciously replenish our microflora.”
Antibiotics are like chemotherapy, in the sense that their deadly effects are indiscriminate; for just as chemotherapy kills many more cells than the targeted cancerous cells, antibiotics kill many more microbes than those that cause sickness or disease. Hence, a person who has taken antibiotics needs to replenish one’s microbial ecosystem, otherwise one may become more susceptible to being invaded by a harmful microbial species in the future. In a similar manner, a plowed or cleared field is more susceptible to being invaded by unwanted plant species because the plants that would normally have grown there and prevented these unwanted species from germinating and taking root, or prevented them from flourishing even if they do germinate, have been eradicated or uprooted by this common, deliberate human activity. For example, burdock and dandelion are two invasive plants that do not do well in a mature ecosystem, since they thrive only when the land has been cleared and all other plants have been removed, whether by mechanical or chemical means.
There is an important difference between cleanliness and sterility, which difference is illustrated by the difference between using soap, which allows one to wash away microbes by binding to them and the bodily substances on which they feed, such as oil, sweat, and dead cells, but without destroying the microbes or altering the ecological balance in which they exist on or inside one’s body, and using antimicrobial soaps, gels, and other substances, which kill microbes indiscriminately, and hence, may unintentionally – and detrimentally – alter the body’s microbial balance. In other words, soap is much less likely than antimicrobial substances to change the composition of a person’s microbial ecosystem.
The implication of this discussion is that, when people try to eradicate all microbes indiscriminately, as those who are frightened of microbes foolishly do, they may unwittingly be upsetting, altering, or impoverishing their bodies’ microbial balance and ecosystem, which may actually make them more susceptible to becoming sick. In other words, having an insufficiency of microbes may be just as bad as having too many, specifically those that cause disease.
We know that persons who live in unsanitary conditions are more susceptible to contracting certain kinds of infectious diseases and illnesses. But those who go too far to the other extreme, because they mistakenly believe that the fewer microbes there are in their environment and on their bodies, the better, and the more healthy they will be, may also be at greater risk of infection. What is true of deadly diseases may also be true of less serious bodily ailments, such as colds and sore throats. A person with a deficient microbial ecosystem may be more susceptible to contracting them than a person who has a flourishing and well-balanced microbial ecosystem. As in many other aspects of life, the goal should be to achieve a healthy balance between these extremes.
If we return to the two cheeses made by Sister Noëlla Marcellino that were both inoculated with E. coli bacteria, the one in her traditional wooden barrel and the other in a microbially-deficient stainless-steel vat, and compare them to people, the one made in the stainless-steel vat is comparable to Modern Industrial Man and Woman, that is, a germophobe, who is obsessed with cleanliness, uses toxic chemical substances to clean one’s residence, takes antibiotics whenever one is sick, uses antibacterial substances, makes sure that everything one eats is either sterilized or cooked until all microbes are destroyed, and would never consider ingesting something that contains a living microbial culture. As a result of all of these precautionary measures, this kind of person is more likely to have a deficient microbial ecosystem. On the other hand, the cheese made in the microbially-rich wooden barrel is like a person who eschews many of these modern industrial products, regularly washes and cleans oneself, one’s clothes, and one’s residence, but without becoming obsessed with cleanliness, avoids using antibacterial substances, and consumes naturally-fermented foods and drinks in order to maintain a healthy and flourishing microbial ecosystem. In addition, one will avoid other common practices, such as consuming very hot foods and drinks, that may also kill or reduce the microbes that exist in parts of one’s body.
Why is it that so many doctors and medical researchers have not been aware of these effects? It is because science proceeds on the basic assumption that all things are best studied and understood in isolation from everything else. Hence, it has taken medical researchers a long time to understand that, besides studying disease-causing organisms in isolation in the highly unnatural conditions that exist in their laboratories, they also need to consider things like the totality of the host organism’s microbial ecosystem in order to understand health and disease. In other words, to the extent that the sterile and unnatural conditions that exist in laboratories differ from the conditions that exist in the real world, the researcher’s understanding of what is actually happening in the real world may be faulty or incomplete.
 I am well aware that this concept is not original, and that many others have discussed it, or discussed something similar to it, but without formulating it as an important natural law.
 Some microbiologists use the word “microbiome” to refer to what I have called by the less elegant phrase “microbial ecosystem.” Although there is nothing wrong with this invented word, firstly, I see no justification for inventing new words when there is no reason for doing so, since this is a primary reason why scientific and other professional papers are incomprehensible to the lay reader; and secondly, by using the term “ecosystem,” which is commonly used primarily in relation to macroscopic species – those that we can see with the naked eye – I wish to emphasize the important balance and interrelations that also exist in healthy, flourishing microscopic communities, features which I do not believe are conveyed by the more neutral word “microbiome,” precisely because it is an invented word, and hence, has no prior meanings or associations.
 This is comparable to the simplistic theory underlying chemical and industrial farming, according to which it is possible to grow only the desired crop while eradicating every other kind of plant from the land. Such practices necessitate the use of greater and greater quantities of harmful pesticides and herbicides, which poison the environment, including water sources that people use for drinking and other purposes. Moreover, it renders these plants and animals weaker and thus more vulnerable to infestation and disease.
 “The Teeming Metropolis of You” by Brendan Buhler. In The Best American Science and Nature Writing 2012. Edited by Dan Ariely. Houghton Mifflin Harcourt, Boston, 2012.
 Cooked: A Natural History of Transformation by Michael Pollan, Part III: A Great White Loaf. Penguin, New York, 2013.
 It was not Darwin but Herbert Spencer, the founder of Social Darwinism, or the application of the theory of evolution to human societies, who coined this phrase.
 Microbe Hunters by Paul de Kruif, pp. 218-219. Harcourt Brace & Company, Orlando, Florida, 1996.
 Cooked, Part IV, Ferment II.
 I suspect the ancient practice of crushing grapes with the feet served another purpose besides releasing the juice contained in the grapes, since all fruits are much more vulnerable to spoilage once their skins have been punctured, damaged, or otherwise breached. Prior to the isolation and use of certain strains of yeast, which are added to crushed grapes by winemakers today to make wine, people had no way of ensuring that the desired yeasts were introduced to the juice and were able to multiply before other undesirable and potentially harmful bacteria colonized it. Since these yeasts and other fungi are probably found on people’s feet, our wine-making ancestors, whether they realized it or not, were introducing them into the juice, thus making its successful fermentation into wine more likely.
 The significant differences in the number of species of bacteria that have been found in different people’s mouths is probably due to differences in people’s hygienic and eating habits. Obviously, those who use antiseptic mouthwashes, brush their teeth regularly, and eat only cooked or sterilized foods that are devoid of microbes will probably have fewer microbial species than those who do not do these things or don’t limit their diet in this manner.
 Not all bacteria require oxygen, as all larger animals do, since some bacteria can function perfectly well in the absence of oxygen.
 For me, this absence of infection is reminiscent of an instance of deduction made by Sherlock Holmes, who, while investigating the case of a stolen horse, remarked about the importance of the guard dog that failed to bark during the night. From this fact, whose significance was overlooked by Watson, Holmes deduced that the dog must have been familiar with the horse thief, which greatly narrowed the list of possible suspects and changed the course of their investigation.
 “The Teeming Metropolis of You” by Brendan Buhler.
 It is also possible that the microbial deficiencies of many people in Western countries may be the cause of the recent rise in allergies in those countries. There are studies showing that children who grow up in highly sanitary, meaning microbially deficient, environments are more likely to develop certain kinds of allergies. Allergies are the result of the body’s reaction, or overreaction, to something in its environment. Just as an alarm system can be oversensitive, triggered by things that pose no threat to the thing it is meant to protect, the human body’s immune system can also be triggered by things that are not harmful to it. In my opinion, this is more likely when the body is deprived of the flourishing microbial ecosystem which has, until very recently, coexisted with humanity during almost all of its existence.
 Of course, the possession of such a microbial colony will not protect one from all infectious diseases, since there are many poor people who do not practice proper sanitation but are nevertheless susceptible to a wide variety of infectious diseases. But it should be remembered that many of these diseases, especially in tropical regions, are transmitted by larger organisms such as mosquitoes, ticks, fleas, and parasites, which can bypass the formidable microbial barrier that is posed by a person’s skin. By being transmitted by larger organisms that can puncture the skin, these microbes are able to gain direct access to the organism’s interior, where they have a much greater chance of successfully overcoming its defenses and multiplying unchecked, since once inside they will find an abundance of the three things they need to thrive – warmth, moisture, and food. There are also many infectious diseases that are transmitted via drinking water, and the fact that many poor people must drink water that is contaminated by fecal remains, lack the means to wash themselves and their clothes regularly, and live in the same place with many other people in overcrowded conditions, also contributes to the spread of certain infectious diseases. This was an important reason why, in the past, some groups of people moved from place to place. While they may have done so primarily to find new sources of food, whether for themselves or for their domesticated animals, after having exhausted the food sources in a certain area, this common practice also separated them from their feces, and thus reduced its potential to transmit diseases.
 Cooked, Part IV: Vegetable.
 I wish to emphasize that I am not saying that people shouldn’t take antibiotics, since they are necessary to kill some disease-causing microbes. But if one does so, it is important that one replenish the microbes in one’s body, otherwise one may become more susceptible to becoming infected by other harmful microbes.